High Performance Wave Plates With Tunable Birefringence Based On All-dielectric And Plasmonic Metasurfaces

Electromagnetic waves exhibit polarization properties owing to the orthogonal oscillations of electric and magnetic fields.This property benefits several applications: biochemical sensing(DNA sensing),photography and holography,optical communications,and wound therapy treatment.In nanophotonics,scholars have been developing light manipulating metamaterial and metasurface structures in ultrathin platforms.Such structures can then be integrated into optical systems and devices,such as flat lenses and wave plates.Conventional approaches to manipulate the polarization state of optical waves employ bulky wave plates,which are designed on multiple layers and using natural birefringent materials.Current trends in research is shifting towards achieving tunable and switchable devices.This thesis reports studies based on designing tunable,birefringent,broadband,and efficient quarter-wave plate metasurfaces in the near-infrared region.Natural crystal such as mica and calcite possess anisotropic refractive indices and can serve as media with linear transmissive birefringences.However,narrow bandwidth and bulk sizes are inherent problems limiting their integration into optical systems.On the other hand,conventional metamaterials suffer from small birefringence tunable ranges and narrow bandwidths—challenges yet to be fully resolved.In this thesis,ultrathin metasurfaces with capacity to intercept,confine,and scatter light with desired polarization states are designed.Birefringence tuning is achieved through manipulating geometry and material properties.Dynamic switching property of the dielectric metasurfaces is demonstrated using an external gate voltage.In general,the metasurfaces have been designed by integrating different materials to obtain the meta-properties required for manipulating phase and amplitude of transmitted light.The primary challenge in metasurface polarization converters revolves around achieving simultaneous control of the phase,amplitude,and polarization properties of light.Previous studies have addressed this challenge through rigorous design methods relying on generation of multiple resonances,each independently tuning a phase range.In this thesis,the hybrid all-dielectric and plasmonic metasurfaces consisting of graphene-silicon and graphene-metal materials,respectively,are designed exhibiting good control of the three aforementioned properties of light.Previous studies have shown that conventional plasmonic metasurfaces have a superior optical response in comparison with their all-dielectric counterparts.However,intrinsic high ohmic losses in metals cause low performances especially in optical frequencies.The hybrid all-dielectric-graphene metasurfaces reported here show remarkable optical polarization conversion.The dielectric elements function as Huygens sources,scattering light at high efficiency,devoid of ohmic losses owing to Mie-resonances and Kerker’s condition.A high polarization conversion ratio reaching 95% was obtained,showing enhanced birefringence performance.Also,in the region of quarter-wave plate performance,wavelength bandwidth of 30% of central wavelength was obtained for the hybrid structures with graphene,compared with 8% for all-dielectric structures without graphene.The ability to confine light into subwavelength dimensions enhances the capacity for manipulating light.Tuning plasmonic birefringence based on surface geometry makes metasurfaces unique compared with the natural anisotropic crystals.Where high nanoscale light confinement is to be used and tunability tailored on propagating surface plasmons then plasmonic metasurfaces should be preferred.By controlling the in-plane properties of the plasmons,the far-field optical properties such as phase and polarization can be manipulated.We have designed metal-graphene quarter-wave plates capable of first intercepting incident light then converting it into localized surface plasmons and then to propagating surface plasmons.As the propagation takes place,the in-plane phase and amplitude are controlled through the properties of metal,periodicity,and the Fermi energy.In addition,a hybrid plasmonic metasurface consisting of dielectric materials embedded inside a film of silver has been designed.Here,the dielectric material acts as a gain media that mitigates the effects of ohmic losses in the metal.This configuration shows properties of lumped circuit elements where the metal functions as a nano-inductor and the embedded dielectric as a nano-capacitor.Broadband quarter-wave plate performance and high degree of birefringence tunability are shown due to integration of the materials.The structure was analyzed using surface plasmon and transmission line theories.